My 14 year old daughter still sleeps with her "security blanket"! And my best friend from high school still had hers (although by then it was more of a rag), and anytime she was really sad or sick she would cuddle with it.
Don't get me wrong, it's not that my 14 year old drags the thing around with her these days but once upon a time she was just like your son -- she took it with her everywhere. When she was about 4 we started putting parameters around when she could have it, mostly because it was getting beat up and because if it got lost we knew we couldn't replace it. So first we said it had to stay in the car when we went to stores, restaurants, etc. Then once she was comfortable with that we gradually moved on to "now it has to stay at home" and then eventually moved to "now you can only have it when you sleep."
Two years ago she went to Alaska for two weeks and I told her leave the blanket because she would be moving around a lot but she said she wouldn't leave it behind but guess what, she did! But luckily we were able to get it back. So now she won't even remove it from the house, even if we go on family vacation. I'm pretty sure she'll still have that thing when she's 100 years old!